This invention is concerned with improved block copolymers. More particularly, it is directed to block copolymers having both polystyrene and low vinyl content polydiene hard blocks to give improved resistance to organic solvents.
Block copolymers of styrene and conjugated dienes such as butadiene and isoprene are well known. Such block copolymers are often selectively hydrogenated so that they contain blocks of polystyrene and blocks of hydrogenated polydiene. The most common configuration for these polymers is the general structure A-B-A where each A is a styrene polymer block and B is a conjugated diene polymer block. These polymers are generally referred to as thermoplastic elastomers because they behave as a vulcanized rubber below their softening point but act as a thermoplastic melt above the softening point and, moreover, even after being raised to such a melt temperature and thereafter cooled, these polymers resume the properties of an elastomer. The diene used to make the polydiene block in these polymers generally must have a relatively high vinyl content, i.e. at least about 25% and preferably up to about 40% by weight because if the vinyl content is lower than that, when the polymer is hydrogenated, the polydiene block is not sufficiently elastomeric in nature.
These polymers exhibit an excellent range and variety of physical properties, including the strength and elastomeric properties described above. However, they have certain limiting characteristics, one of the most serious of which is their sensitivity to organic solvents and particularly to relatively volatile hydrocarbons. Contact with such solvents causes dissolution of the polymer or at least an undesirably high degree of swelling depending on the circumstances and species of the solvents involved as well as upon the particular block copolymers so exposed. It would be highly desirable to eliminate or minimize this solvent sensitivity so as to broaden the utility of these block copolymers.
Several attempts have been made to make slightly different polymers which exhibit improved resistance to solvent attack. Japanese published application JP06306127 describes oil resistant hydrogenated block copolymers which comprise a block of polybutadiene with a vinyl content of above 80% and number average molecular weight of 3,000 to 30,000, an isoprene or isoprene-butadiene block with a vinyl content above 40% and a number average molecular weight of 30,000 to 200,000, and a vinyl aromatic hydrocarbon (styrene) block with a molecular weight of 40,000 to 300,000. These polymers are disadvantageous because the high vinyl or isoprene-butadiene mixed block offer no physical reinforcement in the presence of solvents. Japanese published application 06306128 describes oil resistant hydrogenated 40,000 to 300,000 molecular weight block copolymers of the A-B-A structure, wherein the A blocks are blocks of butadiene with a vinyl content of above 80% and a number average molecular weight of 3,000 to 30,000 and the B block is an isoprene or isoprene-butadiene block with a vinyl content above 40% and a number average molecular weight of 30,000 to 200,000. These polymers are disadvantageous because they are very weak unless the A block molecular weight is 20,000 to 30,000 wherein they are very difficult to manufacture because they have extremely high solution viscosities and again the high vinyl blocks offer no physical resistance to the effects of solvents.
U.S. Pat. No. 3,670,054 describes hydrogenated block copolymers having reduced solvent sensitivity which have the structure C-A-B-A-C wherein each A is a vinyl aromatic hydrocarbon (styrene) block having a molecular weight of 7,500 to 100,000, B is an elastomeric conjugated diene polymer block having a molecular weight of 25,000 to 200,000 and having a vinyl content of 35 to 55%, and each C is a polybutadiene block having a molecular weight between 1,000 and 15,000 and having a vinyl content of less than 25% by weight. These polymers are disadvantageous because they are easily gelled during processing and production by crystallization of both endblocks of the molecule.
The present invention provides block copolymers which have reduced solvent sensitivity and which overcome the foregoing disadvantages. In one embodiment of the present invention, there is provided an asymmetric block copolymer of the structure A-B-C-A wherein each A block is formed of a vinyl aromatic hydrocarbon, preferably styrene, and has a weight average molecular weight of 5,000, preferably 7,500, to 50,000, B is a polybutadiene block having a weight average molecular weight of 1,000 to 15,000 and a vinyl content of less than 25% by weight, and C is an elastomeric conjugated diene polymer block having a weight average molecular weight of 25,000 to 200,000 and has a vinyl content of 30 to 90%, preferably 35 to 80%, and most preferably 35 to 70% by weight. In the second embodiment of the present invention, there is provided a hydrogenated block copolymer of the formula A-B-C-B-A wherein A, B, and C have the definitions set forth above.
The term xe2x80x9cvinyl contentxe2x80x9d refers to the fact that a conjugated diene is polymerized via 1,2-addition (in the case of butadienexe2x80x94it would be 1,2 or 3,4 addition in the case of isoprene). Although a pure xe2x80x9cvinylxe2x80x9d group is formed only in the case of 1,2 addition polymerization of 1,3 butadiene, the effects of 1,2 or 3,4 addition polymerization of isoprene (and similar addition for other conjugated dienes) on the final properties of the block copolymer will be similar. The term xe2x80x9cvinylxe2x80x9d refers to the presence of a pendant vinyl group on the polymer chain. The purpose here is to introduce chain branching and thereby structural irregularity which yields a suppression of crystallinity.
The endblocks of these novel block copolymers are polymer blocks of styrene. Other vinyl aromatic hydrocarbons, including xcex1-methylstyrene, various alkyl-substituted styrenes, alkoxy-substituted styrenes, vinyl naphthalene, vinyl toluene, and the like, can be substituted for styrene and are especially included in this invention. The butadiene used herein must produce a polymer block with a low vinyl content. In other words, the percent of 1,2 addition of the butadiene should be less than 25% by weight, preferably 1 to 10%. When the vinyl content of the polybutadiene block is below 25%, when it is hydrogenated it forms a hard, crystallizable block very similar to polyethylene. It is relatively resistant to solvent attack and assists the polymer as a whole in this regard as discussed in more detail below. The conjugated diene used herein for the internal hydrogenated elastomeric C block must produce a polymer block with a relatively high vinyl content. The percent of 1,2 addition of butadiene or 1,2 or 3,4 addition of isoprene must be in the range of 30 to 90%, preferably 35 to 80%, and most preferably 35 to 70%, because in this range, this polymer block will be elastomeric in nature and thus will give the polymer itself its elastomeric character.
The strength exhibited by the styrene-hydrogenated diene-styrene block copolymers of the prior art is in theory explained by the presence of polystyrene xe2x80x9cdomainsxe2x80x9d which form in the polymer. The polystyrene blocks from different molecules associate together and this physical crosslinking provides the mechanism for strength in styrenic block copolymers. These polystyrene domains, however, are glassy in nature. As such they are susceptible to swelling and dissolution by solvents. Even small amounts of solvents in the glassy polystyrene domains results in significant reductions in physical strength. These strength-forming domains become plasticized.
Crystalline polymers are known to be resistant to dissolution by solvents. While the solvents swell the amorphous segments of partially crystalline polymers like polyethylene, the crystalline segments require heating to lose their structural order and strength.
However, because of crystalline polymers"" strength in the presence of solvents, processing can be difficult. In the case of block polymers having terminal crystalline blocks this difficulty can be severe. During polymerization or processing in solvents, the endblocks can crystallize and lead to gel formation.
The polymers of the present invention exhibit enhanced solvent resistance due to an inherent crystallinity of the polymer while maintaining good solvent processing characteristics.
As discussed above, the polymers of the present invention have the structure A-B-C-A or A-B-C-B-A. The A blocks are formed of polystyrene or some other vinyl aromatic hydrocarbon and have a weight average molecular weight of from 5,000, preferably 7,500, to 50,000, preferably 7,500 to 30,000. If the weight average molecular weight is lower, then the polystyrene domains do not possess enough strength and if the weight average molecular weight is higher, then process viscosities can be prohibitively high. The B blocks are low vinyl content polybutadiene blocks having a weight average molecular weight of 1,000 to 15,000. If the weight average molecular weight is lower, then the melting point and strength of this crystalline block will be too low and if the weight average molecular weight is higher, then processing difficulties due to gellation can result. The C block is an elastomeric conjugated diene polymer block as described having a weight average molecular weight of 25,000 to 200,000. If the molecular weight is lower, then the polymer can be insufficiently rubbery and if the weight average molecular weight is higher, then process viscosities can be prohibitively high.
The molecular weights of linear polymers or non-linked linear segments of polymers such as mono-, di-, triblock, etc., arms of star polymers before coupling are conveniently measured by Gel Permeation Chromatography (GPC), where the GPC system has been appropriately calibrated. For anionically polymerized linear polymers, the polymer is essentially monodisperse (weight average molecular weight/number average molecular weight ratio approaches unity), and it is both convenient and adequately descriptive to report the xe2x80x9cpeakxe2x80x9d (sometimes referred to as xe2x80x9capparentxe2x80x9d) molecular weight of the narrow molecular weight distribution observed. Usually, the peak value is between the number and the weight average. The peak (or apparent) molecular weight is the molecular weight of the main species shown on the chromatograph. For polydisperse polymers the weight average molecular weight should be calculated from the chromatograph and used. For materials to be used in the columns of the GPC, styrene-divinyl benzene gels or silica gels are commonly used and are excellent materials. Tetrahydrofuran is an excellent solvent for polymers of the type described herein. A refractive index detector may be used.
Anionic polymerization of conjugated diene hydrocarbons with lithium initiators is well known as described in U.S. Pat. Nos. 4,039,593 and Re. 27,145 which descriptions are incorporated herein by reference. Polymerization commences with a monolithium, dilithium, or polylithium initiator which builds a living polymer backbone at each lithium site. Typical living polymer structures containing polymerized conjugated diene hydrocarbons are:
X-B-Li
X-A-B-Li
X-A-B-A-Li
Li-B-Y-B-Li
Li-A-B-Y-B-A-Li
wherein B represents polymerized units of one or more conjugated diene hydrocarbons such as butadiene or isoprene, A represents polymerized units of one or more vinyl aromatic compounds such as styrene, X is the residue of a monolithium initiator such as sec-butyllithium, and Y is the residue of a dilithium initiator such as the diadduct of sec-butyllithium and m-diisopropenylbenzene. Some structures, including those pertaining to polylithium initiators or random units of styrene and a conjugated diene, generally have limited practical utility although known in the art.
The anionic polymerization of the conjugated diene hydrocarbons is typically controlled with structure modifiers such as diethylether or ethyl glyme (1,2-diethoxyethane) to obtain the desired amount of 1,2-addition. As described in Re 27,145 which is incorporated by reference herein, the level of 1,2-addition of a butadiene polymer or copolymer can greatly affect elastomeric properties after hydrogenation. In the absence of microstructure modifiers typical anionic polymerization conditions yield 7 to 10% 1,2-addition of butadiene. The 1,2-addition of butadiene polymers significantly and surprisingly influences the polymer as described above. A 1,2-addition of about 40% is achieved during polymerization at 50xc2x0 C. with about 6% by volume of diethylether or about 200 ppm of ethyl glyme in the final solution. A 1,2 addition of about 47% (within the scope of this invention) is achieved during polymerization by the presence of about 250 ppm of ortho-dimethoxybenzene (ODMB) in the final solution. A 1,2 addition of 78%(within the scope of this invention) is achieved during polymerization by the presence of about 300 ppm of 1,2-diethoxypropane (DEP) in the final solution.
The microstructure of the polydiene block has a great effect on the nature of a hydrogenated polydiene block. When the vinyl content is low, a large concentration of diene polymerizes in the 1,4 (head-to-tail) orientation repeatedly. When this is hydrogenated, it looks like polyethylene and has the characteristics of crystalline polyethylene. The concentration of polyethylene crystals decreases with increasing 1,2 addition (i.e. vinyl content) and goes to 0 above about 55% vinyl content.
In general, the polymers useful in this invention may be prepared by contacting the monomer or monomers with an organoalkali metal compound in a suitable solvent at a temperature within the range from xe2x88x92150xc2x0 C. to 300xc2x0 C., preferably at a temperature within the range from 0xc2x0 C. to 100xc2x0 C. Particularly effective polymerization initiators are organolithium compounds having the general formula:
RLi
wherein R is an aliphatic, cycloaliphatic, alkyl-substituted cycloaliphatic, aromatic or alkyl-substituted aromatic hydrocarbon radical having from 1 to 20 carbon atoms.
Suitable solvents include those useful in the solution polymerization of the polymer and include aliphatic, cycloaliphatic, alkyl-substituted cycloaliphatic, aromatic and alkyl-substituted aromatic hydrocarbons, ethers and mixtures thereof. Suitable solvents, then, include aliphatic hydrocarbons such as butane, pentane, hexane, heptane and the like, cycloaliphatic hydrocarbons such as cyclohexane, cycloheptane and the like, alkyl-substituted cycloaliphatic hydrocarbons such as methylcyclohexane, methylcycloheptane and the like, aromatic hydrocarbons such as benzene and the alkyl-substituted aromatic hydrocarbons such as toluene, xylene and the like and ethers such as tetrahydrofuran, diethylether, di-n-butyl ether and the like.
The hydrogenation of these polymers may be carried out by a variety of well established processes including hydrogenation in the presence of such catalysts as Raney Nickel, noble metals such as platinum, palladium and the like and soluble transition metal catalysts. Suitable hydrogenation processes which can be used are ones wherein the diene-containing polymer or copolymer is dissolved in an inert hydrocarbon diluent such as cyclohexane and hydrogenated by reaction with hydrogen in the present of a soluble hydrogenation catalysts. Such processes are disclosed in U.S. Pat. Nos. 3,113,986, 4,226,952 and Reissue 27,145, the disclosures of which are herein incorporated by reference. The polymers are hydrogenated in such a manner as to produce hydrogenated polymers having a residual unsaturation content in polydiene blocks of less than about 1 percent, and preferably as close to 0 percent as possible, of their original unsaturation content prior to hydrogenation. A titanium catalyst such as disclosed in U.S. Pat. No. 5,039,755, which is herein incorporated by reference, may also be used in the hydrogenation process.
The polymers of this invention are useful in compounds which are traditionally made with thermoplastic elastomers. Such compounds include compositions which comprise a block copolymer, from 0 to 300 parts by weight per 100 parts by weight of block copolymer (pbw) of a polymer selected from the group consisting of polyethylene, polypropylene, ethylene-propylene copolymers, polybutylene and poly(ethylene-xcex1-olefin) copolymers, and from 0 to 400 pbw of an oil.
The advantage of the compounded rubber over the neat rubber is that some solvent resistance is gained by use of semi crystalline polymers such as polypropylene. However, the traditional block copolymers described above may still be attacked in the polystyrene blocks by oils, solvents, etc. as discussed above. As a result, compounds made with these polymers have not been suitable for many applications when a compound is to be exposed to these chemicals. When the block copolymers of the present invention are used in place of the traditional block copolymers, these compounds exhibit improved chemical resistance compared to the traditional block copolymers of similar hardness and physical properties. Improvements include reduced weight gain and increased strength retention. These compounds also handle and process easily.